Granular bed filters have at least an inlet screen and in some cases a bed support screen. See U.S. Pat. Nos. 3,410,055 and 3,798,882.
When the temperature of the gas being filtered is high, such as above 1600.degree. F., and/or contains for example 1.4% hydrogen sulfide, the metallurgical requirements prohibit the use of metallic bed support screens or perforated plates. For the same reasons, an inlet screen cannot be used.
The problem solved by this invention is the provision of a granular bed filter lacking an inlet screen and lacking a support screen in a manner whereby the granular bed material is not lost during operation or during backwashing of the granular material which at least in part is subjected to cleaning by fluidization.